1. Field of the Invention
This invention relates to a sliding exhaust gas brake which is connected in the exhaust pipe of an engine in a large motor vehicle, such as a truck or bus, for stopping the flow of exhaust gas to impose an increased load on the engine and thereby exert a braking action on the vehicle especially when it goes down a steep slope.
2. Description of the Prior Art
There is known a sliding exhaust gas brake which is typically constructed as shown in FIG. 2. It comprises a housing 11 in which a slidable valve member 13 is disposed. The housing 11 has a portion defining an exhaust gas passage 12 and connected to an exhaust pipe at both ends 11' and 11" thereof and a portion connected to the exhaust gas passage 12 substantially at right angles thereto and defining a valve chamber 14 in which the valve member 13 is reciprocally movable. The valve chamber 14 has an open end through which the valve member 13 is movable into, or out of, the exhaust gas passage 12. The opposite end of the valve chamber 14 is closed by a wall to which a valve actuator 15 is connected. The actuator 15 comprises a cylinder joined to the end wall of the valve chamber 14, a piston in the cylinder and a piston rod 16 connected to the piston at one end and extending into the housing 11 through the end wall of the valve chamber 14. The valve member 13 is supported on the other end of the piston rod 16. The housing 11 has a valve seat 17 surrounding the exhaust gas passage 12, and on which the valve member 13 rests when closing the passage 12. The valve member 13 has a gas pressure control hole 18 which can be closed by an auxiliary valve member 13'. A sealing member 19 and a bearing 19' are provided in the wall between the valve chamber 14 and the actuator 15 and the piston rod 16 extends through the sealing member 19 and the bearing 19'. A couple of arrows in FIG. 2 show the direction in which exhaust gas flows through the exhaust gas passage 12.
When the valve member 13 is moved to a closing position by the introduction of high pressure gas into the one cylinder chamber remote from the housing 11 or the draw by vacuum suction from the other cylinder chamber, however, exhaust gas having an elevated pressure pushes the valve member 13 and thereby forms a clearance C between the upstream portion of the valve member 13 and the adjacent wall portion of the housing 11. The clearance C establishes communication between the exhaust gas passage 12 upstream of the valve member 13 and the valve chamber 14 and allows the exhaust gas to flow into the valve chamber 14. The exhaust gas contains fine carbon particles and has a high temperature. Those carbon particles and the heat of the exhaust accelerate the aging and wear of the sealing member 19 and the bearing 19'. If they are worn, exhaust gas flows into the actuator 15 and shortens its life. Moreover, the flow of exhaust gas into the actuator 15 gives rise to a delay in the movement of the valve member 13. As a result, the brake fails to function satisfactorily quickly.